Hydraulic tensioners comprising housings with protruding plungers have been used in vehicle engines for maintaining tension in timing chains that transmit rotation from an engine crankshaft to one or more valve-operating camshafts. The plunger usually engages a movable guide having a shoe on which a portion of the chain traveling away from a crankshaft sprocket slides. The plunger is urged by a plunger-biasing spring in the protruding direction, and the plunger and a portion of a plunger-receiving hole in the tensioner housing together form a high pressure oil chamber for containing oil that leaks through a restricted path, usually between the plunger and the wall of the plunger-receiving hole. Leakage of the oil has a damping effect on retracting movement of the plunger, thereby suppressing vibrations while maintaining proper tension.
Tensioners are mounted in various orientations on an engine block, depending on available space and on the presence of other engine parts. These mounting orientations include orientations in which the plunger protrudes upward, or obliquely upward.
U.S. Pat. No. 6,045,471 describes a tensioner having an upwardly protruding plunger. The tensioner has a hydraulic buffer mechanism comprising a hollow, tubular, plunger sleeve formed with a plunger insert bore extending axially through the sleeve. A plunger fits slidably in the plunger insert bore, with its ends projecting axially from the ends of the sleeve. The sleeve is slidable in a plunger sleeve-receiving bore of a housing having an open front end and a closed bottom end. The plunger-receiving bore is formed as a continuation of the rear of the plunger sleeve, and the rear portion of the plunger, projecting from the rear end of the sleeve, is slidable in the plunger-receiving bore. A high pressure oil chamber is formed between the bottom of the plunger-receiving bore and the rear end of the plunger. A check valve incorporated into the rear end of the plunger provides an oil path for flow or oil into the high pressure chamber from an oil supply reservoir inside the plunger. Oil is supplied to the oil supply reservoir from a source external to the plunger housing through an oil supply passage open at a rear portion of the plunger sleeve-accommodating hole in the plunger housing, and an oil supply hole provided on the outer circumferential surface of the plunger.
An anti-retrograde mechanism is provided between the plunger housing and the plunger sleeve to allow only forward displacement of the sleeve with respect to the plunger housing. A plunger spring, provided on the circumference of a portion of the plunger projecting from the front end of the plunger sleeve, urges the plunger in the protruding direction, and a sleeve spring provided between the plunger housing and the plunger sleeve, and having a smaller spring constant than the plunger spring, urges the plunger sleeve forward. When a shock load acts on the front end of the plunger, leakage of oil from the high pressure chamber through a restricted clearance buffers the shock load. The restricted clearance is proved by the sliding engagement between the inner peripheral surface of the plunger-receiving bore and the outer peripheral surface of the plunger.
The above-described hydraulic shock-absorbing tensioner has a greater number of parts than the usual conventional tensioner and a more complicated structure, since it includes a plunger sleeve and a sleeve spring fitted between the plunger housing and the plunger. The overall length of the plunger is necessarily greater than the length of a plunger in the usual hydraulic tensioner. Moreover, even though the plunger is longer than a conventional plunger, the internal space available for storage of oil within the plunger is limited, and is inadequate under some conditions of operation where the demand for oil in the high pressure oil chamber is high.
In the above-described hydraulic shock-absorbing tensioner, the overlap of the mutually sliding surfaces of the plunger housing and the plunger is short, and consequently the leakage of oil through the restricted clearance between these mutually sliding surfaces can become excessive. When the engine is not operated over an extended period of time, oil drainage from the oil reservoir and the high pressure chamber occurs as a result of leakage. Then, when the engine is re-started, the hydraulic damping force exerted as a result of oil leakage cannot be realized until oil is restored in the oil reservoir and in the high pressure oil chamber. As a result, troublesome backlash of the engine timing chain occurs on engine start-up, abnormal sounds are generated, and unexpected damage can occur. Furthermore, when the plunger of the tensioner advances as a result of elongation of the timing chain, the backlash on engine start-up is exaggerated. Similar exaggeration of backlash in the engine timing drive can occur when the tensioner is mounted so that the plunger protrudes upward at a small angle relative to the horizontal.
Accordingly, an object of the invention is to overcome the above-mentioned problems of the prior art, and to achieve one or more of the following advantages, namely: providing a hydraulic tensioner that has an sufficient oil reservoir located within its plunger, ensuring an adequate supply of oil to its high pressure oil chamber upon engine start-up even when the engine has not been operated for a long interval of time; rapidly exhibiting hydraulic damping upon engine start-up; preventing backlash and abnormal sounds; and avoiding a large number of parts.